the following information is provided for those who are more concerned with optimum 
2-way radio performance than they are with convenience of installation for garaging 
and fast-food drive thrus.  the criteria below is offered to this end.  


single antenna installations must meet the following requirements.



#1. must be mounted on the highest possible location on the vehicle.

#2. for more or less omnidirectional operation it must be installed as close to the 
center of the vehicle as possible.  to take advantage of directional characteristics 
simply remember that any counterpoise-dependent antenna radiates the strongest signal 
in the direction in which the longest path of body/frame metal exists.  this is the 
reason why a single antenna radiates more signal favorable for use by truck drivers 
when it is mounted on the passenger side of the cab, not the drivers side.  

#3. in the mounting location selected there should be no body/frame metal that rises 
above the feedpoint of the antenna.  in the case of a lump-loaded base loaded, center
loaded or top-loaded antenna best results will be had when the path to/from the base 
of the loading coil is in the clear and unobstructed as this is the most active portion 
of the antenna.  having said this it is also safe to assume that any body/frame metal 
in the vicinity of the base of the loading coil also greatly affects the tuning and 
feedpoint impedance of the antenna in question.

#4. with reference to feedline length, i have seen many recommending a tuned 1/2 wave 
line be routinely installed.  this should only be done when the measured impedance at 
the feedpoint of the antenna is known to be in the area of 50 - 52 ohms.  remember, 
the reason for using a tuned 1/2 wave line in the first place is to reflect/translate 
the identical impedance at the feedpoint of the antenna to the input of the transmitter.  
since the impedance we want to be seen at the transmitter is 50 ohms then before we 
install a tuned half wave line we had better have some way to measure the impedance 
at the feedpoint of the antenna and we had better make sure it's in the area of 50 ohms.  
if the feedpoint is in the area of 32 - 36 ohms (as it is in most mobile applications) 
the tuned half wave line will actually impede the optimum transfer of energy between 
the transceiver and the antenna.  in this situation you virtually guarantee that a 
match below 1.5:1 will never be realized.  37.5 ohms impedance at the feedpoint for 
example in an otherwise 50 ohm system directly correlates to an swr of 1.5:1.  again, 
the only time a tuned half wave line should be used is when the feedpoint impedance of 
the antenna is known to be +/- 50 ohms.

#5. with reference to #4 above, this rule of thumb generally applies.  in any instance 
where the line is not flat at resonance (1:1) changing the length of the line will 
affect the swr.

#6. many of us can remember the antenna advertising of yesteryear where manufacturers 
like antenna specialists and others showed mobile installations of their products on the 
center rooftops of everyday vehicles.  there is another advantage to rooftop mounting that 
might not be obvious to the uninitiated.  it is common knowledge that when utilizing a 
ground plane design for fixed operation that drooping the counterpoise or ground radials 
at a 45 degree angle below the horizontal plane raises the feedpoint impedance from 32 to 
36 ohms and moves it up to the required 50 - 52 ohms thus providing a much more efficient 
match to 50 ohm coaxial cable.  except for some geometric irregularities along the way, 
the rooftop mounting location is as as close as we can come to simulating these drooping 
radials in a mobile installation.  many of you may have your reasons for not mounting on 
the center rooftop but in all actuality this is the "premium" mounting location for your 
mobile antenna.  the rooftop location also tends to provide a lower angle of incident 
radiation, beneficial to both ground wave and sky wave signal propagation.  the trunk lid 
area is also an excellent choice for those who can't bring themselves to drill holes into 
their roof.  a trunk lid location can also provide a noticably stronger signal to the 
front of the vehicle than to the rear.  

#7. 

A. (from the firestik website) Truck drivers' made co-phase antenna set-ups popular back 
in the early 70's. There are several good reasons for their use. First of all, co-phased 
antennas create a directional pattern that favors communications in front and in back of 
the vehicle. This is ideal for truckers and RV's that use their radios to speak to those 
on the same road/highway that they are traveling on. If you are speaking with someone in 
front or behind your vehicle and either one of you take a turn and leave the more powerful 
RF lobe, a definite change in clarity will be noted. 

B. The second advantage of co-phased antennas is their ability to perform where there is 
little reflective ground plane for the single antenna to radiate its energy from. For 
instance, on fiberglass vehicles or those with light aluminum sheeting over a non-metallic 
frame. In those situations a co-phase set-up allows the antennas to use each other's 
radiation field to direct the combined energy across the horizon. In order to be effective 
at least 2/3's of each antenna must have unobstructed line-of-sight to the other antenna.

C. The third justification for dual antennas involves situations where some or the entire 
signal would be blocked if a single antenna was used. For instances, if you were pulling 
a large trailer. In such cases, having an antenna on both sides of the vehicle prevents 
signal blockage. Note: Drivers that pull large trailers and choose to use one antenna 
will (should) mount the antenna on the passenger side of the vehicle in order to maintain 
contact with vehicles traveling in the opposite direction.

I am in general agreement with all points up to the Note here.  the longest path from a 
drivers side mounting location is right down the rear passenger corner of the trailer, 
across the fog line and off of the passengers front corner, again across the fog line.  
a passenger side mounting location guarantees that the bulk of the radiated signal exists 
in the highway path to the front and the rear of the truck, not being wasted in directions 
across the fog line.  on the other hand, if you're driving in Europe on the other side of 
the road then the drivers side location is the one to use.  same reasons, different lanes.    

D. One additional reason has to do with power. Stock CB's leave the factory with the 
transmitter power limited to 4 watts but there is an underground market of linear 
amplifiers in existence. Inasmuch as power will eventually turn into heat, and the 
antenna will be required to dissipate that heat, two antennas will more than double 
the heat dissipation abilities of a single antenna.

good point. 

E. For general communication, with an Omni-directional radiation pattern, a single 
antenna system is all you probably need.

Note: remember, you can only have an omnidirectional signal as long as counterpoise 
exists for 360 degrees in all directions of the horizontal plane.  ideally the 
counterpoise would extend for at least a quarter wavelength at the lowest operating 
frequency, measured from the feedpoint of the antenna to the extremity.  a single 
antenna on one side or the other of a tractor rig is anything but an omnidirectional 
antenna.  this also applies to antenna installations on conventional vehicles.  again, 
the closest thing to achieving a fully active counterpoise is to locate the antenna as 
close to the highest center of the vehicle as possible.

the dual antenna system made popular back in the 70s is basically a pair of identical 
antennas at a physical spacing of a 1/4 wavelength being fed simultaneously with an 
electrical 1/4 wave phasing harness constructed of quality 75 ohm coaxial feedline to 
provide the necessary parallel impedance transformation.  the pattern generated is 
roughly that of a figure 8 with primary lobes at right angles to the vertical plane 
of the antennas.  contrary to undocumented opinion the directional effect is not 
entirely diminished at closer spacings.  the forward and rearward lobes do recede 
slightly and there is some added response in the side lobes but the overall effect 
is nonetheless still the same.  at the lesser-than-formula spacings i have seen radios 
alone capable of communications over distances in excess of 50 miles in the favored 
direction of the dual antenna systems.


for additional reading....
http://www.firecommunications.com/antennafaq.txt